Sodium-vapor discharge lamp having an envelope with a porous inner wall



United States Patent 3 Claims. (Cl. 206--.6)

ABSTRACT OF THE DISCLOSURE A sodium-vapor lamp having an envelope, the inner wall of which is porous, the pores of the wall opening out at the surface being free of any material and preferably communicating with one another which minimizes selective absorption of the rare gases used in starting the lamp.

The invention relates to an envelope of a gas discharge tube, particularly of a gas discharge tube in which a mixture of two or more rare gases is provided. 1

Such a discharge tube is for example a sodium-vapor discharge lamp, in which the gas filling serves for igniting the lamp. When the lamp is ignited, the temperature rises, so that the vapor pressure of the sodium graduatly increases, after which the emission is maintained for the major part by the sodium-vapor. The gas filling required for the ignition consists of a mixture of rare gases, for example 99% of neon and 1% of argon. Such a mixture of rare gases has a considerably lower ignition voltage than a single rare gas, for example neon. The total rare-gas pressure is chosen as low as possible, since the cmissive output of the lamp decreases rapidly with an increasing pressure.

Such discharge lamps involve the ditficulty that the envelope selectively absorbs argon so that after some time only neon is left. This results in the ignition voltage becoming considerably higher and the lamp can no longer be ignited by the stabilization apparatus provided. There is known a class of borate glass compositions having only a very small degree of argon absorption and having no tendency to color due to the contact with sodiumvapor. These borate glasses, however, have very slight chemical resistivity and can therefore be used only as the inner coating of chemically resistant supporting envelopes. The manufacture of a two-layer envelope requires a special drawing technique and therefore special craftsmanship.

The difficulty of the selective rare-gas absorption from a mixture also arises in gas lasers, for example lasers having a quartz envelope containing a mixture of helium and neon, the neon being absorbed by the quartz envelope.

In the envelope of a gas discharge tube according to the invention the problem of the selective gas absorption is solved in a very simple manner.

The envelope of a gas discharge tube of a glass known for this purpose is characterized according to the invention in that the envelope is coated on the inner side with a layer of porous material, the pores opening out at the surface and being preferably in communication with each other.

For the selective absorption of a rare gas from a mixture of rare gases the discharge is, as a rule, essential and without discharge no noticeable absorption occurs at the operational temperature (a few hundred degrees Centigrade). Owing to the difference in movability of electrons and rare-gas ions, for example, argon ions, during the discharge a potential difference between the glass surface and the positive column of the gas discharge is produced. This potential difference results in that positively charged ions, for example argon ions are absorbed in the glass wall, where they recombine with electrons, the ionization energy being released, for example with argon 15.68 ev. The absorbed gas may disappear from the surface layer by diffusion further into the glass and by desorption, especially thermal desorption, so that it can again take part in the discharge. The balance between these three processesabsorption, diffusion and desorption-finally determines the percentage of rare gas selectively disappearing from the mixture. An important role is played in this respect by the voltage between the electrodes of the discharge tube, the wall temperature and the state condition of the glass surface.

According to the invention it has been found that, when the envelope is provided on the inner side with a layer of porous material the pores opening out at the surface and preferably communicating with each other, the desorption becomes comparatively much greater than the absorption. The conditions are apparently such that, since in such a porous layer an absorbed gas atom after diffusion through the solid substance soon reaches again a boundary, the statistical possibility of desorption is strongly increased as compared with that of absorption.

By measuring the effective surface with respect to the macroscopic surface by :means of adsorption measurement it can be assessed whether a given type of porous layer :fulfills the requirements involved in the object aimed at by the invention. If it appears from such tests that the effective surface is at least twice the macroscopic surface, a distinct effect is ensured or in other terms the selective disapearance of rare gas is reduced. By electronmicroscopic observation of the layer it will be confirmed that the structure of the layer is such that the mechanism described above is performed in a favorable sense.

When the invention is applied to an envelope of a.sodium vapor discharge lamp, the material of the porous layer should be such that it is not colored by the sodium vapor. The glass below said porous layer is slightly protected by the porous layer from the attack of alkali vapor. However, it is preferred not to choose those glasses which exhibit coloring effects to a high extent. Glasses not coloring under the action of alkali vapor are known per se. Hitherto the choice of the material of the inner coating of a sodium vapor discharge lamp has been very restricted, since such a glass must exhibit a low degree of disappearance of argon and not discolor under the action of sodium vapor.

The invention will be described more fuly with reference to the following experiments. The experiments were carried out on a sodium vapor discharge tube, the tubular envelope of which had a length of 30 cms. and an inner diameter of 13 mms. After the electrodes had been sealed in, the tube and the electrodes were degassed by heating in vacuo at 400 C.; then the glas surface and the electrodes were cleaned by burning the lamp with a filiing of pure neon.

The final gas filling consisted of a mixture of 98.9% of Ne and 1.1% of A at a pressure of 9 torr.

The ready gas discharge tubes were burnt for a few hours with a current density of 0.41 A./cm.

The argon absorption was measured by mass spectrometry. Moreover, the argon absorption was measured by determining the ratio of the intensity of a group of lines of the argon spectrum I (3850-4530 A.) and the neon spectrum I (5850-7200 A.) by means of suitable filters (Ilford color filters 601 and 608").

The material of the envelope was the lime glass desig' B303 A120 MgO C80 B30 N830 K20 Glass SiOz (1) A few tubes of lime glass 1, coated on the inner side with a layer of one of the borate glasses 2, 3 and 4, were used for the envelope of sodium-vapor discharge lamps, which were subjected to a duration test; after a burning period of 500 hours, the initial argon content of 1.0% had decreased to 0.6%.

(2) The envelope described under 1 were rinsed for 3 to 30 minutes on the inner side with 0.05 n to 0.1 11 solution of HCl in water. After washing and drying interference colors are produced. The surface is superficially attacked during this treatment, so that a porous structure is obtained. The envelope was tested by burning the lamp. The results were not satisfactorily reproducible. Some- 25 times an improvement was found; the argon content dropped for example from 1.0% to 0.98% after a burning period of 250 hours. Sometimes no improvement was found with respect to untreated borate glass and once even a deterioration was assessed. From observations with an electron microscope it was found that in these cases the leached pores were blocked by material and did not exhibit the required structure.

(3) The envelopes described under 1 were rinsed internally with a solution of 0.5 n of sulphuric acid and boric acid in a concentration approximately equal to the saturation concentration. After drying a porous layer of fine crystalline BaSO is obtained. With this envelope the argon content dropped from 1.1% in sodium vapor discharge lamps after 350 hours of burning only to 1.05%.

('4) An envelope of glass 1 of the above table caused the argon content in a discharge lamp to drop from 1.05% to 0.6% after 150 hours of burning. However, when the lamp was internally wetted with a solution of 2% by weight of colloidal solution of alumina of rod-shaped structure and then dried, it was found that the argon content had decreased to only 0.91% after 150 hours. This alumina, the particles of which had a length of 1000 A. and a diameter of 50 A. is commercially available under the registered trade name of Baymal (Du Pont de Nemours & (30.).

What is claimed is:

1. A glass envelope of a sodium-vapor discharge tube containing a mixture of two or more rare gases, characterized in that the entire inner side of the envelope has a layer of porous material, the pores opening out at the surface being free of material other than said gases and preferably communicating with each other whereby selective absorption by the glass of the envelope of one of the gases is minimized.

2. An envelope as claimed in claim 1, characterized in that the porous layer' consists of alumina particles of rodshaped structure.

3. An envelope as claimed in claim 4 in which the alumina particles have a length of 1000 A. and a diameter of 50 A.

References Cited FOREIGN PATENTS 851,595 10/1960 Great Britain.

MARTHA L. RICE, Primary Examiner. 

